Various methods and apparatus have been proposed for actively reducing vibrations in a region containing a gas or liquid or in a structure of solid bodies. The concept of actively reducing vibrations consists of introducing control vibrations to combine with vibrations in a region or structure so that the resultant vibrations in the region or structure are of a lower amplitude than the vibrations in the region or structure without the control vibrations. The active reduction of audible noise in a region has been particularly pursued, e.g., the reduction of noise in an aircraft cabin generated by jet or propeller engines. Actively reducing vibrations is of considerable importance for low-frequency vibrations because of the difficulty in passively reducing low-frequency components. Passive reduction typically refers to the use of vibration absorbing or blocking materials such as sound absorbing liners in the case of noises in gases. The amount of such vibration absorbing materials needed to be effective increases considerably as the frequency of the vibration is decreased and, thus, is impractical in applications where weight and volume are constrained.
Recently, devices that reduce vibrations in a region or structure by sensing vibrations in the region or structure, decomposing the sensed vibrations into frequency components, calculating output frequency components with some frequency-domain operation, composing control vibrations from the output frequency components, and applying the control vibrations in the region or structure via actuators to reduce the sensed vibrations have been introduced. Generally referred to herein as frequency-domain vibration controllers, such a controller, for example, is disclosed in U.S. patent application Ser. No. 07/575,223, filed Aug. 30, 1990, entitled "Method and Apparatus for Actively Reducing Repetitive Vibrations" by Anders O. Andersson et al. and assigned to the assignee of the present application.
Frequency-domain vibration controllers reduce repetitive vibrations produced by one or more repetitive vibration sources by performing a frequency-domain operation on a present cycle of the sensed vibrations to determine control vibrations and introducing the control vibrations at a later cycle of the sensed vibrations. The control vibrations reduce the sensed vibrations, which consist of the repetitive vibrations introduced by the repetitive vibration sources and the control vibrations introduced by the actuators. The control vibrations can be cyclically updated to increase the amount of reduction.
Current frequency-domain vibration controllers may be used to reduce repetitive vibrations created by multiple sources of repetitive vibrations. Generally, the operation of frequency-domain vibration controllers is synchronized with one of the repetitive vibration sources, referred to herein as the reference source. The repetitive vibrations in the region or structure are sensed synchronously with the reference source, and the control vibrations are applied to the region or structure synchronously with the reference source. Generally, the sensed vibrations are decomposed into frequency components consisting of a fundamental frequency and harmonics thereof. The fundamental frequency of the decomposition is chosen to be the fundamental frequency of the reference source. A frequency-domain vibration controller operating synchronously with a reference source can effectively reduce the repetitive vibrations produced by multiple sources of repetitive vibrations if all sources of repetitive vibrations operate at exactly the same frequency. However, there are applications in which there are multiple sources of repetitive vibrations operating at slightly different frequencies. In these applications, the slight differences in the frequencies of the sources produce vibrational beats that are not reduced by the frequency-domain vibration controller.
Take, for example, the application of a frequency-domain vibration controller for reducing the noise in an aircraft cabin generated by the aircraft's jet engines. Prior art frequency-domain vibration controllers used in aircraft were operated synchronously with the rotational frequency of one of the aircraft's jet engines, i.e., the chosen reference source. However, the jet engines of an aircraft rarely operate at exactly the same rotational frequency. Therefore, each jet engine produces a repetitive vibration of a slightly different frequency. The differences in the frequencies of the repetitive vibrations produce vibrational beats that are not effectively reduced by the frequency-domain vibration controller. These vibrational beats are annoying to the passengers of the aircraft.
The present invention improves prior art frequency-domain vibration controllers such that these controllers can more effectively reduce repetitive vibrations generated by sources operating at slightly different frequencies. In essence, a frequency-domain vibration controller operating in accordance with the present invention operates synchronously with the repetitive vibration source chosen as the reference source, but frequently corrects the control vibrations based upon the instantaneous phase differences between the sources of the repetitive vibrations.
Generally, in frequency-domain vibration controllers, the control vibrations are cyclically updated to approach waveforms that optimize the reduction of the repetitive vibrations in the region or structure. In addition, some frequency-domain vibration controllers incorporate an adaptive method of updating the control vibrations. Such adaptive methods effectively optimize the reduction of the sensed vibrations whether or not changes are occurring in the repetitive vibrations, the region or structure, or the frequency-domain vibration controller. The method of the present invention can be used with such adaptive frequency-domain vibration controllers. Further, the method of the present invention is adaptive itself.